1. Field of the Invention
The present invention relates to an overcurrent detection device for detecting an overcurrent of a load circuit which performs the on/off control of a load by using a semiconductor switch.
2. Description of the Related Art
For example, a load such as a lamp or a motor mounted on a vehicle is coupled to a battery (DC power source) via a semiconductor switch, whereby the load is controlled so as to be driven and stopped by turning on and off the semiconductor switch, respectively.
In such a load circuit, an excessive current flows through a coupling electric wire when a short circuit occurs. Thus, upon the detection of the generation of an overcurrent, it is necessary to immediately turn the semiconductor switch of f thereby to protect the circuit entirely.
Thus, there has been proposed a circuit which is arranged in a manner that a voltage generated between the two terminals of the semiconductor switch is measured, and the generation of an overcurrent is detected when the measured voltage exceeds an overcurrent determination voltage set in advance thereby to turn the semiconductor switch off.
FIG. 2 is a circuit diagram of a related art showing the configuration of a load circuit including an overcurrent detection device. As shown in this figure, a semiconductor switch (FET101) is provided between a load 101 and a battery VB. The drain of the semiconductor switch (FET101) is coupled to the positive electrode side terminal of the battery VB and the source (voltage V2) thereof is coupled to the one end of the load 101. Each of the other end of the load 101 and the negative electrode side terminal of the battery VB is grounded.
The load 101 includes a resistance component RL and an inductance component LL.
The gate of the semiconductor switch (FET101) is coupled to the output terminal of a driver circuit 102 via a resistor R103. Numerical values described at the lower sides of the symbols R101 to R103 of the resistors represent concrete resistance values of these resistors, respectively.
The positive electrode side terminal of the battery VB (the drain of the FET101) is grounded via a series circuit of the resistors R101 and R102. Thus, supposing that the voltage of the battery VB (the drain voltage of the FET101) is V1, the voltage at the coupling point (hereinafter, this voltage is referred to a reference voltage V4) between the resistors R101 and R102 equals to a voltage obtained by dividing the voltage V1 by the resistors R101 and R102. The reference voltage V4 is supplied to the negative side input terminal of a comparator CMP101, whilst the source voltage V2 of the semiconductor switch (FET101) is supplied to the positive side input terminal of the comparator CMP101.
Supposing that the voltage between the drain and the source of the semiconductor switch (FET101) is V DS, the drain current thereof is ID and the on-resistance of the semiconductor switch (FET101) is Ron, the following expression (1) is obtained.V DS=V1−V2=Ron·ID  (1)
Supposing that the difference (V1−V4) between the battery voltage V1 and the reference voltage V4 is the overcurrent determination voltage, when the voltage V DS between the drain and the source exceeds the overcurrent determination voltage (V1−V4), the output signal of the comparator CMP101 is inverted, whereby the overcurrent is detected. When the generation of the overcurrent is detected, a turn-off instruction signal is supplied to the driver circuit 102, whereby a drive signal having been supplied to the gate of the semiconductor switch (FET101) is stopped thereby to turn the semiconductor switch (FET101) off.
In other words, supposing that the on resistance Ron of the semiconductor switch (FET101) is constant, when the current flowing into the load 101, that is, the drain current ID increases, the voltage V DS between the drain and the source increases. When the voltage V DS between the drain and the source exceeds the overcurrent determination voltage (V1−V4), the semiconductor switch (FET101) is tuned off thereby to protect the circuit.
In order to prevent that the drain current (the current flowing through the load 101) which is a current normal state, is erroneously determined as an overcurrent, the overcurrent determination voltage (V1−V4) is set to a value lager than the maximum value (hereinafter referred to as V DSmax) of the voltage V DS between the drain and the source in the normal state. That is, in order to prevent that a large current generated in the normal state such as a rush current generated at the time of turning-on of a power supply is erroneously determined as an overcurrent, the overcurrent determination voltage (V1−V4) is required to be set to be larger than the maximum V DSmax of the voltage V DS between the drain and the source generated in the normal state.
Further, in view of the facts that the comparator (CMP101) has an offset voltage±Voff peculiar to elements and that there is a voltage variation A due to the variance of the respective parts constituting a control circuit, the overcurrent determination voltage (V1−V4) is required to be set so as to satisfy the following expression (2).(V1−V4)>V DSmax+Voff+A  (2)
Thus, the overcurrent determination voltage (V1−V4) is inevitably set to a value lager than the voltage V DS between the drain and the source generated in the normal state.
However, when the overcurrent determination voltage (V1−V4) is set to a value lager than the voltage V DS between the drain and the source generated in the normal state, in the case where a short-circuit occurs and so an overcurrent flows actually, it takes a long time to turn the semiconductor switch (FET101) off thereby to protect the circuit.
As described above, in the overcurrent detection device of the related art, in order to prevent that a large current such as a rush current generated in the normal state is erroneously determined as an overcurrent, the overcurrent determination voltage (V1−V4) is required to be set to a large value. On the other hand, in the case where an overcurrent is generated due to a short-circuit etc., in order to detect this phenomenon immediately to turn the semiconductor switch (FET101) off thereby to protect the parts and electric wires, there is a demand that the overcurrent determination voltage (V1−V4) is required to be as close as possible to the voltage V DS between the drain and the source generated in the normal state. These requirements are antinomy and so there is a problem that it is not easy to satisfy these requirements simultaneously.